A Brief Introduction to the High-Frequency Welded Steel Pipe Process.
In high-frequency welding, a steel strip is fed into a forming machine to form a cylindrical billet. Passing through an induction coil, the magnetic field near the coil generates an induced current that flows through the edges of the steel strip. The edges of the steel strip are heated by resistance due to their own resistance. The heated edges are then pressed by extrusion rollers to form a weld. High-frequency welding involves no additives; it is essentially a forging weld. If the production process is well controlled, there will be no residual molten metal or oxides on the fusion surface. This is due to the heat generated by the high-frequency current entering from the ends and edges of the steel strip. The heat-affected zone is slightly darker than the base metal because carbon diffuses towards the heated edges of the steel strip during welding, and is absorbed at the edges as the weld cools. Especially near the edges, carbon oxidizes to CO or CO2, leaving the iron without carbon, resulting in a lighter color. The metal flow lines seen on metallographic specimens are actually the flat, discontinuous planes formed when the high-carbon zone of the billet is rolled into the steel strip. The angle of inclination of the metal flow lines is often used to evaluate the degree of upsetting during welding.
Common Defects in High-Frequency Welded Steel Pipes
High-frequency welded steel pipes may exhibit various defects, and the terminology for these defects is not entirely standardized. Based on the characteristics of our high-frequency welded steel pipe production, the following defects frequently occur: ① Inclusions; ② Insufficient fusion; ③ Adhesive welding; ④ Cast welding; ⑤ Porosity; ⑥ Skip welding. While not all of these defects are present, they are frequently observed in high-frequency welded steel pipes.
Inclusions – Black Overheated Inclusions
This type of defect occurs when metal oxides are not extruded with the molten metal and are trapped on the molten surface. It forms on the surface of the molten metal at the V-nose. At the V-nose, if the approach speed of the steel strip edge is less than the melting speed, and the melting speed is higher than the molten metal discharge speed, a narrow fan-shaped area containing molten metal and metal oxides forms after the apex of the V-nose. These molten metals and metal oxides cannot be completely discharged through normal extrusion, thus forming an inclusion band. After the weld is flattened, black overheated inclusions are easily visible on the weld fracture surface. Compared to the fibrous fracture surface of the weld, the fracture surface of black overheated inclusions is flat and lacks metallic luster. These defects may occur singly or in chains. The probability of inclusions increases when the V-cut angle narrows, for example, to less than 4° or when the silicon-to-manganese ratio in the steel is less than 8:1. However, the silicon-to-manganese ratio in the steel is more difficult to control than other influencing factors, mainly depending on the chemical composition of the base metal.
Prevention measures for black overheated inclusions:
① Control the V-cut angle between 4° and 6°;
② Ensure a stable V-cut length using reliable tooling equipment;
③ Achieve better weld quality with a relatively low welding temperature;
④ Avoid having a silicon-to-manganese ratio in the base metal of the steel strip that is less than 8:1.
Inclusions – White Overoxide Inclusions
Calling this type of defect “white overoxide inclusions” is not entirely accurate. It is actually insufficient fusion caused by pre-arc welding, without any foreign matter trapped on the fusion surface. Typically, burrs or rust form a bridge before reaching the apex of the V-cut, causing a short circuit and resulting in current fluctuations that produce a pre-arc phenomenon. The short-circuit current instantaneously changes the current direction, reducing the heat of the V-cut. Instantaneous current generates very small defects, generally not exceeding the wall thickness in length. From the weld fracture surface, a small, bright, flat plane surrounded by a fibrous fracture surface can be seen.
Prevention measures for white peroxide inclusion defects:
① Control the V-not angle between 4° and 6°;
② Reduce edge burrs;
③ Proper edge treatment or reduce edge damage to the steel strip;
④ Keep the cooling water clean and prevent it from flowing into the V-notch.
Insufficient fusion—Incomplete edge fusion (cracking): The edges of the two steel strips are not completely fused to form a good weld. The cracked edges are blue, indicating that the steel strip has been heated. However, if the steel strip edges are flat and smooth, it indicates incomplete weld fusion. The most direct cause of this type of defect is insufficient welding heating. However, considering other related factors, such as the weld heat input, the V-not angle, and V-not heating length, the installation and cooling conditions of the magnetic rod, and the size of the induction coil, these factors can individually or in combination cause defects.
Measures to prevent inadequate fusion:
① Match the welding input to the material properties and welding speed;
② Position the magnetic rod 3.2–3.5 mm beyond the center of the extrusion roller;
③ The V-notch angle should not exceed the pipe diameter;
④ The V-notch angle should not exceed 6°;
⑤ The difference between the inner diameter of the induction coil and the outer diameter of the steel pipe should not exceed 6.5 mm;
⑥ The strip width should be suitable to meet the requirements of the produced steel pipe diameter.
Inadequate fusion – Inadequate edge fusion (edge waviness)
Inadequate edge fusion is caused by the absence of metal on the fusion surface. This type of defect often appears on the outer or inner edge of the strip and is similar to peroxide defects. This defect is caused by the weld flattening and cracking at the 3 o’clock position. The fracture surface is flat and dull.
Preventive measures for inadequate edge fusion:
① Ensure the strip edges are straight and parallel;
② Use a better extrusion amount;
③ If the defect is caused by bulging, the fracture surface is silver-gray; use a larger welding heat input.
Insufficient Fusion – Insufficient Central Fusion (Internal Cold Welding) After a weld with insufficient fusion fails, the cross-section in the middle of the wall thickness appears as a flat, silvery-gray strip. The edges are fibrous. This welding defect occurs when the welding speed requires power exceeding the rated power amplifier of the welding machine, and the entire edge of the steel strip does not have sufficient time to heat to the optimal temperature and depth required for the weld. Insufficient central fusion can also be caused by incomplete removal of molten metal from the joint surface.
Preventive Measures for Insufficient Central Fusion:
① Increase welding machine power;
② Increase welding extrusion amount;
③ Increase V-gap length or reduce welding speed.
Cast Welding (Brittle Welding) Cast welding occurs when molten metal on the joint surface is not completely removed. The cast metal on the fusion surface contains metal oxides, similar to overburned oxides. The fracture morphology varies depending on the content of residual cast metal. Currently, most exhibit a flat, brittle morphology. Metallographic examination reveals cast metal on the joint surface. Cast-welded high-frequency-welded steel pipes crack when flattened.
Preventive measures for cast welding:
① Increase weld venting;
② Increase steel strip width.
Porosity (Pinholes)
Porosity on the weld joint surface is caused by insufficient venting during high-temperature welding. The fracture morphology is fibrous, with spherical bright white spots randomly distributed throughout the fracture. When white spots appear on the outer wall, their surface appearance turns black due to oxidation. Small pores can be seen before external burr removal, and pores can also be seen on the fusion line after external burr removal.
Preventive measures for porosity:
① Reduce welding input;
② Increase extrusion amount.
Skip Welding
Typically, this type of defect has a regular, continuous distribution. Defects on the outer side of the steel pipe wall resemble wavy defects, generally appearing at equal intervals.
Preventive measures for skip welding:
① Add welding current filtering equipment;
② Check input voltage;
③ Check rollers and shafts.
Suggestions for preventing defects
In actual production, defects are often caused by the combined effect of several factors. A narrow V-shaped opening does not necessarily produce overburnt oxides unless the extrusion amount is slightly less than a normal value. Small extrusion volumes may be caused by a slightly narrower slitting width of the steel strip, wear on the tooling, or improper equipment installation. Welding defects can also arise from factors outside the weld zone; for example, cold welding may occur due to cavitation in the cooling pump, preventing sufficient cooling of the magnetic rod. The magnetic rod heats up instantly, reducing the heat concentrated at the V-notch and causing cold welding. Increasing the welding input can prevent cold welding defects before all cooling pumps malfunction and the magnetic rod becomes completely ineffective. The best way to prevent defects is to identify their root cause and collect as many possible operational parameters as possible. Determine relevant parameters such as working width, welding speed, screen current, screen pressure, grid current, and extrusion volume, observe actual operation, and record data, focusing on identifying abnormal fluctuations to analyze the causes of defects. During production, set values may slightly exceed normal values, but if several related variables simultaneously exceed slightly, the cumulative result is sufficient to cause defects. Analyzing and summarizing common defects and their causes during production is highly beneficial for experienced operators, improving efficiency and reducing costs.
Summary
(1) Most welding defects are caused by improper installation or commissioning of the unit.
(2) Selecting a reasonable manufacturing plan, monitoring daily operation records, and regularly training high-frequency welders are beneficial to reducing defects.
(3) Improving the quality of edge trimming and edge treatment, as well as the coil energy storage process, is beneficial to reducing edge loss defects.
(4) Pre-maintenance can prevent tooling wear.
Post time: Oct-31-2025